The present invention relates to methods and a device for determining dimensional properties of a measurement object having a plurality of geometric elements.
EP 2 738 515 A1 discloses a typical example of such a device in the form of a coordinate measuring machine. The coordinate measuring machine has a workpiece receptacle and a CCD camera, which is movable in three orthogonal spatial directions relative to the workpiece receptacle. The CCD camera is part of a measuring head that can be used to determine the position of selected measurement points on a measurement object relative to a reference coordinate system. Usually, such a device is used to determine measurement point coordinates for a plurality of measurement points on a measurement object. On the basis of said measurement point coordinates, it is then possible to determine dimensional properties of the measurement object, such as, for instance, the diameter of a bore or the distance between two edges on the measurement object.
In principle, it is possible to manually control the movements of the measuring head relative to the measurement object and the recording of the measured values. However, an automated measurement sequence is desirable in quality control of industrially produced products, such that a plurality of measurement objects of identical type can be measured rapidly and as reproducibly as possible. Creating an automated measurement sequence requires fundamental knowledge of the functioning of coordinate measuring machines and experience of how different geometric elements on a measurement object can best be measured. Furthermore, an optimum measurement sequence may vary depending on what kind of measuring head and/or what movement axes are available on a coordinate measuring machine. In this regard, a measuring head having a non-contact sensor, as in the case of the coordinate measuring machine from EP 2 738 515 A1, for instance, may require a different measurement sequence than, for example, a tactile measuring head, i.e. a measuring head having a probe element configured to touch selected measurement points on the measurement object.
EP 2 738 515 A1 proposes to use a wide-angle monitoring camera in addition to the CCD camera on the measuring head. The wide-angle monitoring camera is configured to record an image of the entire measurement object from a bird's eye perspective. This (further) image is shown on the display of an operator terminal in order to make it easier for the operator to generate an automated measurement sequence taking into account and avoiding possible collisions between the measuring head and the measurement object. However, the device from EP 2 738 515 A1 does not offer more extensive support in the generation of a measurement sequence and the operator thus requires profound knowledge and experience in order to generate an optimum measurement sequence for a specific measurement object.
Under the brand name CALYPSO, the assignee of the present invention (Carl Zeiss Industrielle Messtechnik GmbH) offers software for generating an automatic measurement sequence and for processing the measurement results obtained. The basic principles of CALYPSO are described for example in a brochure entitled “Einfach Messen and was Sie dazu wissen sollten—Eine Fibel der Messtechnik” [“Straightforward measurement and what you should know to implement it—Primer for metrology”] (order number from Carl Zeiss: 61212-2400101). The measurement sequence is generated by CALYPSO on the basis of so-called test features. A test feature represents a dimensional property of a defined geometric element (measurement element) on a measurement object such as, for instance, the diameter of a bore, the roundness of a cylinder section or the position of such a geometric element relative to the reference coordinate system. In order to quantify a test feature with respect to a geometric element, it is generally necessary to detect a plurality of measurement points on the geometric element, such that the quantification of a test feature may require the recording of a plurality of individual measured values. With the selection of a test feature, CALYPSO generates control commands that can be used to automatically control the measuring head for measuring the associated measurement points. By using test features as a basis, CALYPSO makes it easier for an operator to generate the measurement sequence, because the test features generally correspond to information which the operator can infer from a technical drawing of the measurement object. Furthermore, CALYPSO is able to automatically identify geometric elements having defined standard geometries, such as, for instance, circle, cylinder, rectangle, straight line, etc., if the corresponding measurement point coordinates of the geometric element are available for evaluation.
In order to generate an automatic measurement sequence, however, the operator has to identify and define all the test features on the measurement object beforehand, that is to say that generating the measurement sequence begins “at the desk” with the identification and definition of all required test features. Afterward, the operator has to assign all the geometric elements on the measurement object to the previously selected test features. The operator then performs—by manual control—all the measuring steps, one after the other, that the machine is intended to perform later in an automated manner. Even though CALYPSO already facilitates the generation of an automatic measurement sequence in this way, the operator must still have considerable technical knowledge and experience in order to generate a meaningful measurement sequence. The operator's knowledge and experience must include, in particular, the individual functioning of the coordinate measuring machine used in order to obtain a measurement sequence which yields all measured values with the desired accuracy as efficiently as possible.
Under the designation “Model Series IM”, Keyence Corporation offers a measuring system in which measurement sequences are generated by means of a symbol menu. Here, too, the operator firstly selects general test features from a symbol menu. Afterward, in a further step, the operator selects measurement elements at which the previously selected test features are intended to be determined. The measured values are recorded on the basis of silhouette images of the measurement object. Even though the selection of test features on the basis of offered symbols for general test features is very convenient for the operator, this known system, too, requires considerable knowledge and experience of the relationship between individual measurement elements on a measurement object and the test features of interest.